AC installation causing furnace to hit high limit

Posted by Jorge on November 13, 2007, 4:12 pm

Hello,
I have a 25 yr old Olson gas furnace. Our house has never had AC until
this summer, when I had one installed.
Now, the furnace is hitting the high limit and cycling off. The tech
said that my outflow duct is quite small, and that
the AC coil has reduced air flow even further, causing the furnace to
overheat. The company suggests the following options:
1) new furnace: higher blower pressure would solve overheating issue
2) modify ductwork by adding a bypass around the coil for winter usage
3) increase return air flow somehow
I was not informed of any of these issues before the installation.
Does anyone have an opinion about the best option. Also, when scoping
out a house for AC, is it standard procedure
to look at the size of the duct work?
Thanks!!
Jorge

Posted by CJT on November 13, 2007, 5:35 pm

Jorge wrote:
show/hide quoted text
First consider whether the blower is on low speed in heating mode.
Often they're on low for furnace and high for AC. It might be very
simple to rewire it so that it runs on high speed in heating mode, too.
--
The e-mail address in our reply-to line is reversed in an attempt to
minimize spam. Our true address is of the form che...@prodigy.net.

Posted by Noon-Air on November 13, 2007, 8:37 pm

show/hide quoted text
25 year old, Jurassic furnace?? Why are you even screwing with it?? Call a
*competent* contractor, get a complete "room-by-room" Manual J heat
load/lloss calculation, and a Manual D, ductwork calculation done to find
out exactly what your home *NEEDS*, then go from there.
Right now all your doing is putting band-aids on the system. Get it done
right, and the savings on your utility bills alone could pay for the new
system in as little as 5 years. The lowest bidder is *NOT* the way to go....
nor is the cheapest equipment. You *WILL* get what you pay for, likewise,
your not gonna get what you don't pay for.
Before you ask about brands, the best brand is the one that is correctly
sized and installed. Brand names?? try consumer reports.

Posted by Stormin Mormon on November 13, 2007, 8:59 pm

Since the furnace worked for however many years without the AC coil, the
bypass makes a lot of sense.
As another poster suggested, check and see if the blower has multiple
speeds. Typically high for AC, and medium or low for heat. His suggestion
was workable, use high fan for heat, also.
Third option. Duct in another heat run, and some more return duct. Increase
the physical size of, or numbers of ducts and vents. Since there's not
enough duct, put more in.
--
Christopher A. Young;
Hello,
I have a 25 yr old Olson gas furnace. Our house has never had AC until
this summer, when I had one installed.
Now, the furnace is hitting the high limit and cycling off. The tech
said that my outflow duct is quite small, and that
the AC coil has reduced air flow even further, causing the furnace to
overheat. The company suggests the following options:
1) new furnace: higher blower pressure would solve overheating issue
2) modify ductwork by adding a bypass around the coil for winter usage
3) increase return air flow somehow
I was not informed of any of these issues before the installation.
Does anyone have an opinion about the best option. Also, when scoping
out a house for AC, is it standard procedure
to look at the size of the duct work?
Thanks!!
Jorge

Posted by udarrell on November 13, 2007, 10:37 pm

This is a multi-part message in MIME format.
--------------090505020104050303050705
Content-Type: text/plain; charset=us-ascii; format=flowed
Content-Transfer-Encoding: 7bit
Jorge wrote:
show/hide quoted text
The contractor &/or tech should have evaluated the ductwork & airflow
delivery conditions & informed you of the apparent problems prior to the
installation of the AC coil. It might require all three options above &
more! (?)
Look at the furnace info data tag for the Blower horse power & the ESP
static pressure rating.
It might have a mere 1/6th HP blower motor & the ESP Rating of the
airhandler may be well below 0.50" WC.
If the above is the case, I would replace the furnace because when
operated in the cooling mode it will NOT perform well & could possibly
result in damage to the compressor.
Also, the ductwork may be totally inadequate for air conditioning mode.
The ductwork system must be designed to provide ample airflow in "the
cooling mode which normally requires considerable more airflow (esp. in
southern hot climate states) than the heating mode." Follow manual D
for proper ductwork design to match the airflow requirements of your AC
system & proper air delivery to each room!
Study these pages & then find a contractor that can do the job right!
http://www.udarrell.com/proper_cfm_btuh_duct_sizing_air_conditioning_systems.html
http://www.udarrell.com/external_static_pressure_readings.html
The link below is for those with similar "Oil furnaces with added air
conditioning." - Do it right!
Check the blower curve graph or chart!
http://www.udarrell.com/oil_furnace_heating.html
These are only suggestions, you are responsible for what you do or have
done, NOT me!
- udarrell
--
WISDOM PRINCIPLED EMPOWERMENT COMMUNICATIONS -
THE REAL POLITICAL ISSUES & WISDOM Principled PEOPLE EMPOWERMENT
http://www.udarrell.com/ (Updated Again)
It is time to create a "Department Of Peace - HR 808)" that will preempt the
Department of War, camouflaged as the Department of Defense.
http://www.udarrell.com/my_pages2.htm
(* My Airconditioning Links, Hunting Shooting, Angus Cattle)
Reality Is Not An Easy Thing To Be Confronted With, or to ACCEPT.
--------------090505020104050303050705
Content-Type: text/html; charset=us-ascii
Content-Transfer-Encoding: 7bit
show/hide quoted text
Jorge wrote:
<blockquote
cite="mid1194988345.248761.77290@19g2000hsx.googlegroups.com"
show/hide quoted text
I have a 25 yr old Olson gas furnace. Our house has never had AC until
this summer, when I had one installed.
Now, the furnace is hitting the high limit and cycling off. The tech
said that my outflow duct is quite small, and that
the AC coil has reduced air flow even further, causing the furnace to
overheat. The company suggests the following options:
1) new furnace: higher blower pressure would solve overheating issue
2) modify ductwork by adding a bypass around the coil for winter usage
3) increase return air flow somehow
I was not informed of any of these issues before the installation.
Does anyone have an opinion about the best option. Also, when scoping
out a house for AC, is it standard procedure
to look at the size of the duct work? Thanks! Jorge
show/hide quoted text
should have evaluated the ductwork & airflow delivery conditions
& informed you of the apparent problems prior to the installation
show/hide quoted text
Look at the furnace info data tag for the Blower horse power & the
show/hide quoted text
It might have a mere 1/6th HP blower motor & the ESP Rating of the
show/hide quoted text
If the above is the case, I would replace the furnace because when
operated in the cooling mode it will NOT perform well & could
show/hide quoted text
The ductwork system must be designed to provide ample airflow in "the
cooling mode which normally requires considerable more airflow (esp. in
show/hide quoted text
ductwork design to match the airflow requirements of your AC system
show/hide quoted text
Study these pages & then find a contractor that can do the job
show/hide quoted text
<a
show/hide quoted text
<a
show/hide quoted text
The link below is for those with similar "Oil furnaces with added air
show/hide quoted text
<a
show/hide quoted text
These are only suggestions, you are responsible for what you do or have
show/hide quoted text
WISDOM PRINCIPLED EMPOWERMENT COMMUNICATIONS -
THE REAL POLITICAL ISSUES & WISDOM Principled PEOPLE EMPOWERMENT
<a class="moz-txt-link-freetext"
show/hide quoted text
It is time to create a "Department Of Peace - HR 808)" that will preempt the
Department of War, camouflaged as the Department of Defense.
<a class="moz-txt-link-freetext"
show/hide quoted text
(* My Airconditioning Links, Hunting Shooting, Angus Cattle)
Reality Is Not An Easy Thing To Be Confronted With, or to ACCEPT.
show/hide quoted text
--------------090505020104050303050705--

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